This guide will show you how to get the best ending, and get certain collectibles and weapon/health upgrades. (this guide will NOT explain the story, because the story explains itself in-game)
1. --Start Point--
You start in a hollow cave with a monitor that has a heart on it, a floppy disk, a door, and some water. The monitor is a Life Refill. Press down when on it to refill your health. The floppy disk is a Save Point.
Don't stay in the water too long, because you only have so much air.
Save your game, then jump up to the door (enter the door by pressing down).
1a. --First Cave--
You are now outside the hollow area. Going right does no good; there are blocks in the way, and an enemy is there (you currently have no weapon!), so go left (watch out for the spikes, they'll hurt you!).
Avoid the bats and grab the Life Capsule (press down when on it).
Go southeast, and then enter the tiger's mouth.
Press down when on the chest, and you've gotten your first weapon, the -Polar Star-!
After a brief cutscene, head back out the door. You can now shoot enemies! Some enemies (depending on your level and weapon) will take more hits to defeat. After defeating an enemy, it can either spawn EXP (orange triangles; bigger ones yield more EXP) or Health (flashing hearts; 3 in a cluster yields more health points).
(an explanation on the level up/exp system will be made shortly)
Head back to the door you first came out of, and save your game.
In part 2, we will continue onwards to the next destination.